This application claims the priority of Korean Patent Application No. 2004-3803, filed on Jan. 19, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
Exemplary embodiments of the present invention relate to digital television (DTV).
2. Description of the Conventional Art
Digital broadcasting may be achieved using, for example, satellites, ground waves, or cables as transmission media. These transmission media have their respective characteristics, and based on these characteristics, different bandwidth and modulation methods may be used.
In ground wave digital broadcasting, the frequency band may be, for example, ultra high frequency/very high frequency (UHF/VHF). In order to maintain compatibility with analog broadcasting, bandwidth may also be the same, or substantially the same, as conventional broadcasting. Transmission formats may use a higher bit transmission rate over a reduced bandwidth through multi-level modulation techniques such as a vestigial sideband (VSB) method.
A digital TV (DTV) signal may be transmitted with a weaker strength compared to the analog TV signal strength. DTV signal standards may include a variety of coding methods and channel equalization methods.
FIG. 1 is an example diagram of a VSB data frame format containing data and a synchronization signal. Referring to FIG. 1, a frame may include two fields, and each field may include 313 data segments. A data segment may be formed of 832 symbols.
The first 4 symbols of a data segment may be a segment synchronization part such as a horizontal synchronization signal, and a first data segment in a field may be a field synchronization part such as a vertical synchronization signal.
FIG. 2 illustrates an example of the composition of the field synchronization part, which may include the segment synchronization part of 4 symbols, pseudorandom number sequences, VSB mode related information of 24 symbols, and 104 symbols, which may not be used. The PN511 sequence may be formed of 511 pseudo-random symbols. The second PN63 sequence of 3 PN63 sequences may be inverted in each successive field. A logic level ‘1’ may transition to logic level ‘0’ and logic level ‘0’ may transition to logic level ‘1’. Based on the polarity of the second PN63, fields may be divided into even fields and odd fields.
At a broadcasting station, before transmitting a signal, the signal may pass through a mapper, which may change the signal to a desired power level. When broadcasting 8 VSB, an output level of a mapper may be one of 8 level symbol levels (amplitude levels). According to a VSB standard, a horizontal synchronization signal of 4 symbols may be generated and inserted into every 832 symbols. The horizontal synchronization signal (1, 0, 0, 1) may have two levels, and may be repeated (e.g., continuously) in each data segment.
FIG. 3 illustrates an example of the frequency characteristic of a broadcast signal defined in DTV standards. Referring to FIG. 3, fc, the center frequency, may indicate a central frequency in 6 MHz bandwidth of each ground wave channel, and fp, the pilot frequency, may indicate a frequency in which a carrier wave of a transmission signal may exist. In a demodulating receiver, the location of a pilot frequency (fp) may be restored and converted into a baseband signal. A carrier wave may be recovered using a frequency and phase locked loop (FPLL). Carrier wave synchronization may depend on a pilot signal and it also may be more sensitive to the distortion of a pilot signal.
FIGS. 4 and 5 illustrate example estimation curves of frequency and phase offsets, respectively, due to distortion of a pilot signal. Referring to FIG. 4, carrier wave offset tracking performance may be degraded more significantly in a negative frequency range. A negative frequency estimation range, for example, when the size of the pilot signal may be reduced by 50%, may be reduced to within −50 kHz, which may be about 25% of the range when the size of the pilot signal may be 100%.
The example phase offset estimation curve of FIG. 5 illustrates that the size of a pilot signal may be reduced by 50%, and the linear interval may remain the same, or substantially the same, as when the size of the pilot signal may be 100%.